1. Field of the Invention
The present invention relates to a small-size image forming device utilizing electrophotography, such as a copying machine, a printer or a composite machine. More specifically, it relates to an image forming device with improvements for solving problems which occur when using a substantially spherical toner.
2. Description of the Related Art
In an image forming device utilizing the electrophotography method, an image is formed by forming an electrostatic latent image on a surface of an image holding member comprising an organic photoreceptor of a drum or belt shape, or the like, by a known electrophotographic process, developing the electrostatic latent image with a toner so as to obtain a toner image, electrostatically transferring the toner image onto a recording paper directly or via an intermediate transfer member, and fusing the toner onto a surface of the recording paper by heating, or the like.
A dry toner, in which a colorant, a charge controlling agent, or the like is dispersed into a resin that is the main component, and which is caused to take a particulate form as needed, is mainly used as the toner. Regardless of whether a toner is specified as a one-component developing agent or a two-component developing agent, most such dry toners are produced by the so-called mechanical pulverization method including steps of homogeneous dispersion by kneading a colorant, or the like, into a resin that is the main component, mechanical pulverization and classification so as to obtain desired particle size and particle distribution.
In view of the recent demand for high image quality, toner of reduced particle size and narrow distribution of particle size is desired for use in such image forming devices. When the distribution of particle size is wide, the ratio of toner having a small particle relative to toner having a large particle size, or vice versa, is increased so as to generate the following problems. In the case of toner wherein the amount of small particles is large, the toner tends to scatter from the developer thereby contaminating the interior of the image forming device, or the like. Also, in the case of a two-component developing agent, since the toner can easily adhere to a carrier, the toner charge property is deteriorated. In contrast, in the case of toner wherein the amount of large particles is large, there are problems such as a tendency for image quality deterioration, or the like.
However, in the case in which a toner of small particle size and narrow particle size distribution is produced by the above-mentioned method of mechanical pulverization, the production ability and the yield are drastically lowered, thereby increasing the cost. Therefore, as a method for producing such a toner, wet methods such as polymerization and dissolution have been proposed.
In the polymerization method toner particles are obtained via a polymerization reaction and granulation of a combination of a monomer and a colorant, or the like. Since the particle size can be controlled by adjusting the reaction time, or the like, theoretically it is said that extremely narrow particle distribution is possible.
Moreover, in the dissolution method toner particles are obtained via preparation of an oil phase by dissolving or dispersing a binder resin and a colorant, or the like, in an organic solvent, and suspension granulation of the oil phase component in a water phase. In this production method a reduction in particle size and control of particle size distribution control can be achieved.
It is characteristic of a toner obtained via the wet methods such as polymerization and dissolution to have a substantially spherical particle shape. In contrast, a toner obtained by the above-mentioned mechanical pulverization method generally has an amorphous particle shape. Therefore, compared with the amorphous toner obtained by the mechanical pulverization method, it is known that toner produced via the wet methods has the advantage of extremely improved transfer efficiency since the substantially spherical toner of small particle size has a small contact area with the surface of the image holding member whereby the adhesion force of the toner with regards to the surface of the image holding member is small. Due to such a high transfer efficiency and the fact that less toner is wasted, it is possible to reduce the amount of toner used in comparison with conventional toners, thereby making it economical and environmentally friendly.
However, since toner obtained by the wet methods has a substantially spherical particle shape, it is known to have the following disadvantage. When either toner remaining on the surface of the image holding member after transfer in an image forming device without a cleaning device, or toner remaining on the surface of the image holding member after passing through a cleaning step in an image forming device having a cleaning device, passes between contacting portions of a contact charger and an image holding member, the toner is deformed so as to adhere to the surface of the image holding member. When this occurs, repetition of the adhesion results in toner filming, wherein the toner becomes fixed to the surface of the image holding member as a foreign substance.
When the image formation is executed after toner filming has occurred, residual images or stripes are generated on the obtained image deteriorating the image quality. In the case where a contact charger such as a charge roll with a small diameter is used for providing a small image forming device, improvements with respect to both charge failure and the above-mentioned toner filming have yet to be achieved.
In particular, in the case where a spherical toner of small particle size obtained by the wet methods is used, it has been extremely difficult to prevent generation of toner filming in a conventional image forming device having a cleaning device such as a cleaning blade or in an image forming device without a cleaning device wherein residual toner is collected by the developer.
For example, in an image forming device having a cleaning device such as a cleaning blade, since a small-particle spherical toner cannot be cleaned sufficiently, the toner passes under the blade. Therefore, the above-mentioned toner passes between the contacting portions of the image holding member and the contact charger. At the time, the toner is deformed by the contact charger and thereby adhered to the surface of the image holding member. Therefore, due to repetition of the adhesion, the toner is fixed on the surface of the image holding member generating the so-called toner filming and having the adverse effect on image quality.
Moreover, in an image forming device without a cleaning device wherein the residual toner is collected by the developer, the toner remaining on the surface of the image holding member after transfer passes between the contacting portions of the surface of the image holding member and the contact charger. At the time, the above-mentioned toner is deformed by the contact charger and thereby adheres to the surface of the image holding member. Therefore, due to repetition of the adhesion, the toner is fixed on the surface of the image holding member generating the so-called toner filming and having adverse effects on image quality.
An object of the present invention is to solve the above-mentioned problems. That is, an object thereof is to provide an image forming device capable of preventing generation of toner filming while obtaining a stable image quality without defects over a long period, and which is environmentally friendly, by restraining an amount of a spherical toner remaining on a surface of an image holding member after transfer and deformed as it passes through contacting portions of the image holding member and a contact charger in the case of forming an image using the spherical toner.
The above-mentioned object can be achieved by the present invention described below. That is, according to a first aspect, the present invention provides an image forming device comprising an image holding member, a contact type charging means for charging a surface of the image holding member by making contact therewith, an exposing means for forming an electrostatic latent image by exposing the surface of the image holding member charged by the contact-type charging means according to image information, a developing means for developing the electrostatic latent image via a spherical toner so as to provide a toner image, and a transfer means for electrostatically transferring the toner image from the surface of the image holding member to a transfer material, wherein a toner shape change ratio (Tt) of deformed toner particles passed between contacting portions of the image holding member and the contact charger, expressed by the following formula (1), is within a range of 50 to 100 percent:
xe2x80x83Tt(%)=(h/x)xc3x97100xe2x80x83xe2x80x83Formula (1)
In the formula (1), x denotes a maximum length (xcexcm) of a deformed toner particle projected image, h denotes a maximum length (xcexcm) of the deformed toner particle projected image formed on a surface perpendicular to an axis in the maximum length direction of the deformed toner particle projected image, and xxe2x89xa7h.
According to a second aspect, the present invention provides an image forming device, wherein a shape index (SF) of the spherical toner, expressed by the following formula (2), is 135 or less:
SF=(2xcfx80L2/4A)xc3x97100xe2x80x83xe2x80x83Formula (2)
In the formula (2), L denotes the maximum length (xcexcm) of the spherical toner particle projected image, and A denotes an area (xcexcm2) of the spherical toner particle projected image.
According to a third aspect, the present invention provides an image forming device, wherein a volume average particle size of the spherical toner is within a range of 2 xcexcm to 9 xcexcm.
According to a fourth aspect, the present invention provides an image forming device wherein the contact charger is a charge roll, comprising a rotating member, at least one intermediate layer disposed on a surface of the rotating member, and an elastic member provided further on the surface, and a diameter of the charge roll is in a range of 6 mm to 13 mm.